Purifying light metal



Patented Aug. 9, 1932 UNITED STATES PATENT OFFICE JOHN A. GANN, OF MIDLAND, MICHIGAN, ASSIGNOR TO THE DOW CHEMICAL GOM- PANY, OF MIDLAND, MICHIGAN, A CORPORATION OF MICHIGAN PURIFYING- LIGHT METAL iio Drawing.

As is well known, crude magnesium, such as electrolytic cell metal, is often relatively impure, the impurities consisting largely of non-metallic ingredients, and Where the metal to be used is remelted scrap magnesium metal and/ or its alloys, it is equally important that steps be taken to eliminate foreign matter, if clean castings are to be secured either in the form of stock bars or finished articles.

One such method is described in my co-pending application Ser. No. 651,431, filed July 13, 1923, now Patent- No. 1,754,788.

The object of the present invention is the provision of a further improved method for v eliminating such impurities either in the case of magnesium metal or of so-called-light metal alloys in which magnesium is thepredominant ingredient. i

To the accomplishment of the foregoing 2 and related ends the invention, then, consists of the steps hereinafter fully described and particularly pointed out in the claims, the following description setting forth but one of the various'ways in which the principle of the invention may be carried out.

The main step in my improved process consists in the washing or thorough intermixture of the metal or such alloy thereof while in molten condition with a suitablejfiuxing material. Several different fluxes have been.

.35 which becomes excessive where the molten metal is exposed directly to the atmosphere. Anhydrous'magnesium chloride, which is one of thematerials employed in the electrolytic production of the metal, serves also when in 4C} molten condition as a flux or protective coating but has too high a melting point when used alone so that sodium chloride is ordinarily added to lower such melting point. The flux thus composed of magnesium chloride and sodium chloride is disclosed in U. S. Patent N'o. 1,37 7 ,374 and also in German Patent No. 122,312, the latter describing a'fluX consisting of equimolecular weights ofmagnesium chloride, sodium chloride and potassium chloride with the addition of a small Application filed October 4,1928. Serial No. 310,440.

amount of calcium fluoride. U. S. Patent No. 1,476,192 discloses a flux consisting of magnesium and sodium chlorides in approximately equal proportions with 'a relatively small amount of barium chloride added to raise the specific gravity of the flux slightly above that of the metal, either magnesium alone or an alloy thereof, with which it is to be used. A different type of flux is disclosed in U. S. Patents, Nos. 1,576,080 and 0 1,661,526, which describe the use of a small amount of flux to which is added various com pounds so that it will be thick and pasty while the metal is being purified.

In the practice of my present improved method of purification, any suitable flux may be employed in which the proportion of ingredients is such that the melting point of the mixture is lower than that of the metal or alloy and at the same time such flux mixture, when in molten state, is limpid and mobile, and has a surface tension such as to cause it to coat the exposed surface of the metal. Such flux, as for instance that men- I tioned in the said Patent No. 1,476,192, will also preferably have a slightly higher specific gravity than the metal or alloy so that the latter will float in the relatively large bath of flux to be used, in other words, it is proportionately large when compared with the so volume of metal it floats and coats. When using a flux of this type I have found that in general the coefficient of expansion ofthe molten flux is greater than that of molten metal. Thus the higher the temperature of the molten bath is raised, the more the flux will rise to form an increasingly thicker protective film on the upper surface-of the metal. At sufliciently high temperature the ratio of densities is even reversed, so that the metal procedure, the flux employed. at this stage may in some cases be that resulting from a subsequent step in the purification treatment as will presently appear. As soon as the metal is in proper fluid state, the flux of course being likewise fluid at the temperature involved, the metal is gently agitated or stirred with the'flux so as to be thoroughly washed therewith without dispersion of the metal therein. At the same time the temperature of the molten bath may be raised, e. g.,

casting temperature. If the metal were relatively clean at the start of the process, this one purification will be all that is necessary to prepare the metal for casting. However, if the metal were relatively dirty at the start, a still further purification step will sometimes be necessary. In such case, when the metal has cooled back to the desired temperature it is dipped or otherwise transferred into another pot containing afresh lot of substantially pure flux whereit is again gently agitated or stirred in the-fashion just described and allowed to stand. This retreatment with clean flux I find will remove substantially all of the remaining non-metallic impurities from the magnesium or its alloy and the metal will be then in suitable condition for casting.

The purified body of metal, or at least the major portion thereof, obtained as a result of the last step, is transferred at suitable temperature directly from the pot to the molds, leaving behind the flux with the impurities taken up thereby from such body of metal. This flux is then used in repeating thefirst step wherein the crude metal with maximum content of impurities is melted down and agitated as hereinbefore set forth.

None of the metal from the second step that .is'left in the flux'bathis of course lost since it is mingled with the raw metal added in repeating the first step.

In order to ladle the metal from one pot to the other, the thin film of the flux that overlies the surface of the molten mass of metal in the pot is broken and brushed to one side so as to uncover the clear metal and the ladle can then be filled and lifted before the film' is -re-established. However, where proper precautions are taken, such re=establishment of the film will occur before any oxidation or other harmful effect is produced on th exposed metal andby careful manipulation the operator is at the same time able .toavoid taking up any of the flux in the ladle. It

will be understood, of course, that the method of purification-in hand is not limited to any such particular method of transferring the metal. It will also be understood that I I may increase the number of successive operations wherein progressively cleaner flux is employed instead of operating simply in one or two stages as described.

While the molten metal and flux may be continually agitated or stirred while being heated to a temperature as high as 300 C. above its melting point, I-have found that it is generally sufficient to gently and thoroughly agitate or wash the metal with the flux at approximately the highesttemperature attained. At this point the metal and flux are both very limpid and mobile, and a thorough washing of the former is easily attained.

The term light metal as used hereinwill be understood to designate either magnesium by itself or alloys of the character herein; before described wherein magnesium is the predominant constituent. Similarly, term scrap will be understood as designating any. form of such light metal, whether shavings, turnings, chippings, fragments of castings, or the like, with which objectionable foreign matter is in any way intermixed or included. r 1 v Other modes of applying the principle of "my invention may be employed instead of the one explained, change being made as regards the method herein disclosed, provided the step or steps stated by any of the following claims or the equivalent of such stated means be employed.

I therefore particularly point out and distinctly claim as my invention 1. The method of purifying magnesium and alloys thereof which comprises melting the metal in a proportionately large bath of a mobile molten flux, temporarily subjecting the molten mixture of metal and flux to a.

the I temperature of about 100 to 300 C. above ing temperature, and separating the metal from the flux. I

1 2. The method of purifying magnesium and alloys thereof which comprises melting the metal in a proportionately large bath of a mobile molten flux, temporarily sub ecting the molten mixture of metal and flux to a temperature of about 100 to 300 C.

above the melting point of the metal, whereby to increase the fluidity of such flux, agitatnon-metallic impurities are removed from the metal by the washing action of the flux, allowing the mixture to stand quletly to seging the molten metal with the flux whereby regate the flux and the metal, while cooling to acasting temperature, separating the metal from the flux, and repeating the foregoing treatment with another body of flux.

3. The method of purifying magnesium and alloys thereof which comprises melting the metal in a proportionately large bath of a mobile molten flux containing magnesium chloride, temporarily subjecting the molten mixture of metal and flux to a temperature of from 100 to 300 C. above the melting point of the metal, agitating the molten metal with the flux, allowing the mixture to stand quietly to segregate the flux and the metal while cooling to a casting temperature and separating the metal from the flux.

4;. The method of purifying magnesium and alloys thereof which comprises melting the metal in a proportionately large bath of a mobile molten flux containing magnesium chloride, temporarily subjecting the molten mixture of metal and flux to a temperature of from 100 to 300 C. above the melting point of the metal, agitating the molten metal with the flux, allowing the mixture to stand quiet- 1y to segregate the 'flux and the metal while cooling to a casting temperature, separating the metal from the 'flux indrepeatin the lffloregoing treatment with another bo y of Signed by me this 2nd day bf October, 1928. i a

JOHN A; GANN. 

